214 



MORPHOLOGY 



jsh /<r~¥^X (belonging to the inner fleshy layer) 



// \ (=§5^ /lmWKk\\ ^eing suppressed. This is just the 



(i I ^JfT •' '* ' reverse of tin- ■ axular condition in 



^vj' 474 475 476 Ginkgo, in which the set of vascular 



Figs. 474-476. -Microsporophylls(sta- strands belonging to the outer 



mens) of Taxaceac: 474, Podocarpus (after fleshy layer is suppressed, and the 



Hooker); 475. Taxus (after Richard); ^ ^longing to the inner fleshy 

 476, Torrcya (after Hooker). — From . 



Engler and Prantl. layer is developed. In early stages 



of the ovule the nucleus is entirely 

 free from the integument, but as the ovule 

 develops largely from beneath, the region 

 in which the nucellus and integument are 

 free from one another is carried to the tip 

 of the ovule. A notable change in the 

 ovule, as contrasted with the preceding 

 groups of gymnosperms, is that there is 

 no nucellar beak or pollen chamber (fig. 

 477); nor is there developed about the 

 megaspore mother cell any special diges- 

 tive (glandular) zone of cells, as in 

 Ginkgo. The development of the linear 

 tetrad and the selection of the innermost 

 megaspore for functioning are as usual. 



Female gametophyte. — The female ga- 

 metophyte develops by the usual stages : 

 free nuclear division (up to 256 nuclei), 

 parietal wall formation, and centripetal 

 growth. It is important to note the 

 variation in the appearance of the arche- 

 gonial initials. In some cases they are 

 not differentiated until the gametophyte 

 has become quite extensive; that is, they 

 appear comparatively late in its history. 

 In other cases, however, notably in 

 Torreya, the archegonium initials appear 

 as soon as wall formation has filled the 

 embryo sac with tissue (figs. 478, 479). 



Arciiegonia. — The archegonia range 

 in number from one to eleven, the neck 



Fig. 477. — Longitudinal sec- 

 tions of ovulate strobilus of Tor- 

 reya at different ages, showing 

 the enveloping bracts and the soli- 

 tary terminal ovule; upper section 

 (the older) shows the two integu- 

 ments (it, 01), nucellus (n), and 

 dec]) placed mother cell. — Aftei 

 Coulter and Land. 



